1. Technical Field
The present invention relates to the metrology of optical elements, and in particular, the measurement of thickness, optical power and optical aberrations of lenses.
2. Description of Related Art
In the manufacturing of lenses, obtaining fast and accurate measurements of lens dimensions is a challenging problem. This is particularly the case for small low cost high volume lenses, such as contact lenses for the eye. Low-coherence interferometry (LCI) is one measurement technology that may be applied to this measurement problem.
LCI has applications in many fields from medical imaging to glass manufacturing. The low-coherence interferometry is based on using a light source with a very short coherence length. The light is split between two arms of an interferometer and then recombined and directed onto a detector. Interference occurs when the path lengths of the two arms of the interferometer are equal to within the coherence length of the light from the source.
There are numerous known configurations of such interferometers, such as the Michelson, Mach-Zehnder, and Fizeau interferometers, and others described in the text, Principles of Optics: Electromagnetic Theory Of Propagation, Interference and Diffraction of Light, M. Born and E. Wolf, Cambridge University Press, Cambridge; New York, 1999, 7th ed. Another example of such an interferometer is described in U.S. Pat. No. 6,724,487 of Marcus et al., “Apparatus and method for measuring digital imager, package and wafer bow and deviation from flatness,” the disclosure of which is incorporated herein by reference. (“Marcus '487” subsequently herein.)
The interferometer disclosed therein by Marcus '487 is based on the use of piezo fiber stretching technology as the means of changing the optical path-length. A narrow beam of low-coherent light is directed onto the surface of the test object. It is common to focus the beam inside or in proximity to the test object. The reflected light from all of the object interfaces, which the beam traverses, is then collected and analyzed by the interferometer. The interferometer is used to extract the optical distances between the interfaces. The physical distances are obtained by dividing the optical distances by the group refractive indices of the material which makes up the space between the interfaces.
In a typical application, the light beam is directed along the optical axis of a lens. The axial thickness of the lens is then obtained by dividing the measured optical distance by the group refractive index of the glass or plastic material of the lens. Such measurement represents a point measurement, since only the distance between the two points (point of entry and exit of the measurement beam) is measured, while the information about the rest of the object (lens) is unknown.
When using LCI, it is possible, in principle, to move the measurement beam laterally with respect to its axial propagation, and to measure the thickness of the object (lens) at different locations. However, this approach is associated with difficulties, arising from the LCI requirements. One such requirement is to orient the measurement beam perpendicularly to the interfaces, to maximize the collection efficiency of the reflected beam. Not only is this difficult to do when just one interface is present, but in the case of two or more non-parallel interfaces (such as in a lens) such a requirement cannot be fundamentally satisfied. For most lenses, the only locations in which the two lens surfaces are parallel and able to be positioned perpendicular to the measurement beam are near the center of the lens. In order for the LCI to be able to measure effectively, the reflected light coming back from the lens must be within the numerical aperture of the lens and optical fiber. For most lenses, only the central region of the lens can be measured by using LCI. This is insufficient for the characterization of many lens products.
A wavefront sensor is a device for measuring the optical aberrations of an optical wavefront. This is accomplished by measuring the irradiance and phase distribution of the light beam at a particular plane in space. Although there are a variety of wavefront sensing technologies, including lateral shearing interferometers, curvature sensors, pyramid wavefront sensors, Focault knife-edge test, Ronchi test, and Shack-Hartman Wavefront Sensor (SHWFS), the SHWFS has been the most frequently employed, since it is capable of measuring both irradiance and phase distributions in a single frame of data.
U.S. Pat. No. 5,936,720 by Daniel R. Neal et al. entitled “Beam Characterization By Wavefront Sensor” issued on Aug. 10, 1999 and U.S. Pat. No. 6,130,419 by Daniel R. Neal “Fixed Mount Wavefront Sensor” issued on Oct. 10, 2000 describe the basics principles of operations of a wavefront sensor employing a two dimensional Shack-Hartman lenslet array; the disclosures of these patents are incorporated herein by reference. Further details on the use of Shack-Hartman wavefront sensors in optical metrology may be found in “Application of Shack-Hartmann wavefront sensing technology to transmissive optic metrology” by R. R. Rammage et al., Proc. SPIE Vol. 4779, Advanced Characterization Techniques for Optical, Semiconductor, and Data Storage Components, pp. 161-172, (2002).
U.S. Pat. No. 7,583,389 by Daniel R. Neal et al. entitled “Geometric Measurement System And Method Of Measuring A Geometric Characteristic Of An Object” issued on Sep. 1, 2009, describes a white light interferometer to measure surface curvature and or thickness of an object. This patent discloses the requirement of tilting of the object with respect to the interferometer apparatus and measuring at a variety of tilt angles in order to characterize a single surface of the object. The disclosure of this patent is incorporated herein by reference.
U.S. Pat. No. 7,623,251 by Daniel R. Neal et al. entitled “Geometric Measurement System And Method Of Measuring A Geometric Characteristic Of An Object” issued on Nov. 24, 2009 describes the use of wavefront sensing to measure surface curvature of an object on one or more surfaces. The measurement requires moving the object relative to the measurement apparatus and measuring at a variety of positions and/or angles in order to characterize the curvature of the one or more surfaces. The disclosure of this patent is incorporated herein by reference.
The disclosures of these patents notwithstanding, there remains an unmet need for a measurement apparatus and method that enables the non-contact measurement of lens or other optical element thickness and surface curvature of the top and bottom surfaces across a broad range of locations on the lens surface, along with the measurement of the optical aberrations of the lens or other optical component without the need of moving the lens or optical element with respect to the measurement apparatus during measurement. There also remains an unmet need to be able to measure the physical dimensions and optical performance parameters of multifocal and toric lenses. Such a measurement would inherently be faster since the sample would be static or not moving during the entire measurement procedure.